The lock cylinders art is requiring higher and higher security and there is a need in the art for the development of a shape or profile of a cross section of key and corresponding shape of the keyway in the cylinder plug to accommodate a hierarchical lock providing high security. The shape of the keyway is the first barrier that rejects or filters an unauthorized key in attempt to operate the lock cylinder.
There is only a finite space in a lock cylinder plug that can be occupied by the key and that space must be structured to allow for the maximum number of unique keyway shapes to be able to develop lock systems of adequate size. In large modern lock systems it is usual to arrange the structure of the keyways in a manner so that at least three levels of a hierarchical system can be provided, with one master key blank at the top level of the system, some sub-master key blanks at a medium level and several change key blanks at the lowest level of the hierarchical system. A new key section design must be different from prior key sections so that the key blanks can be controlled by the manufacturer and the end user can benefit from the security offered by the exclusivity of this key control via the key blanks.
Unique key profile shapes provide for additional protection against unauthorized key copying. Most key blanks of the generally flat rectangular key profiles are manufactured with single pass formed milling cutters that shape the side of the key blank. The axis of rotation of the cutter is held parallel to the side of the blade. Keys using an undercut groove profile require additional form cutting on specially designed machines that are usually not available at commercial duplicator operations and thus the blanks are more difficult to copy or counterfeit.
Early in the development of lock cylinders, it became apparent that there were specific parameters that affected the size of the lock cylinder systems that could be developed and that there were many design factors that influenced the wear of the key and the cylinder and thus the longevity of the system. Key blanks were designed with these parameters in mind. Representative examples of the prior art include the following:
In U.S. Pat. No. 0,263,244, Taylor discloses a key blank design that offers an economically simple solution to the problem of having a key that moves too freely in the keyhole. This offers a very minimal keyway shape in the plug and key profile in the blank.
In U.S. Pat. No. 0,420,174, Taylor teaches a unique but limited master keying technique that uses a Y shaped key section in a plug that allows two differently shaped key profiles to contact their own areas of the non rotating tumbler pins.
In U.S. Pat. No. 0,567,305, Donavan discloses a method of expanding the number of key sections, thus increasing the available size of lock systems, by dividing the key blank height into various areas and using consistent warding techniques at these locations to develop hierarchical keyways or key profiles. This increases the system size of pin tumbler cylinders. The bittings of one key can be repeated on a different key blank, configured with a different key profile, and the cylinders into which these individual keys fit can also be operated by a higher level key designed to insert into both of the keyways.
In U.S. Pat. No. 0,608,069, Noack discloses an arrangement of key section warding that provides improved wear on the key and the key contact area on the tip of the locking pins. In addition it provides a narrow cross sectional width under the bitting area, thus making it difficult to manipulate pick tools under the tumbler pins.
In U.S. Pat. No. 3,499,304, M. Noujoks teaches a method of designing key section warding where both faces of the keys are provided with alternating ridges and grooves. It utilizes a master key blank that has all the grooves of the series but not the ridges, while the key blanks of a lower hierarchical level have varying ridges.
In U.S. Pat. Nos. 4,168,617 and 4,368,629, Prunbauer discloses more methods of designing key section warding where the master key will fit into the subordinate keyways but the lower keys will not fit into the master keyways. In one embodiment, the ridges and grooves defining the key section are of a rectangular cross-section shape, and the outwardly projecting variable ridge on the subordinate key extends laterally beyond any of the other variable ridges. The subordinate key is thicker at its further ridge than the master key is at any location. In another embodiment the master key is formed of a zigzag shape, that is with its opposite sides formed of a plurality of planar facets each of which is substantially parallel to a respective planar facet on the other side.
In U.S. Pat. No. 4,416,128, Steinbrink teaches another unique method of designing key sections where the longitudinal grooves on both sides of the key blank are formed with bottom faces that lay substantially along the arc of a circle.
In U.S. Pat. No. 4,653,298, Tietz discloses a method of designing master key section warding that incorporates an invariable or family profile near the bitting area on the blank, and the variations defining the individual key sections are located near the spline or bottom edge of the blank. Additionally there are at least two profile formations that cross a center line in the key blank, one ridge is extending beyond the surface of the blank, and the variations are made with longitudinal grooves having rectangular cross sections.
In U.S. Pat. No. 4,683,740, Errani illustrates a key section design that has a undercut groove shape making it very difficult to manipulate a pick tool in the keyway of the plug. The undercut groove is formed by means of cutters having their rotational axis inclined in relation to the sides of the key blank.
In U.S. Pat. Nos. 5,715,717 and 5,809,816, Widen teaches some very specific methods of designing key sections using a three sided undercut groove located closest to the bottom edge of the key blank and extending inwardly inclined towards the bottom of the key blank, or using an undercut groove with a substantially flat surface which is inclined towards the groove bottom surface.
In U.S. Pat. No. 6,145,357, Stefanescu teaches a method of designing master key section warding that utilizes a key blank with a T-shaped cross sectional area with all the profile ribs having specific curvilinear cross sectional contours, with rounded front and flank portions.
In U.S. Pat. No. 6,851,292, Kruhn discloses a method of designing lock and key warding that incorporates specific perpendicular groove surfaces on one side of the key section, and slanting surfaces on the other side that are positioned in a relationship designed to trap, or limit the motion of a picking tool inserted into the key way.
While the prior art has developed usable key sections, they fail to maximize the area of the plug and do not allow for the development of many large master keying systems.